The invention provides a method for obtaining the stray capacitance of an AC motor. The method includes: establishing a common-mode equivalent circuit when the stator and the rotor of the AC motor aregrounded; based on the common-mode equivalent circuit, establishing a relational expression among the stray capacitance of the AC motor, a common-mode voltage, a shaft voltage, and stator and rotor ground impedances; measuring the common-mode voltage and the shaft voltage of the AC motor under different stator and rotor grounding impedances, and calculating the stray capacitance inside the AC motor by using the relational expression based on the measured common-mode voltage and shaft voltage. The method can effectively calculate the stray capacitance inside the motor by measuring the common-mode voltage and the shaft voltage under three groups of different stator and rotor ground impedances, can take account of the influence of the motor speed, temperature rise and the like to obtain thestray capacitance inside the AC motor in an actual running state.